The present invention pertains to motion compensators used in connection with offshore drilling and production operations. One type of motion compensator is commonly referred to as a "drill string compensator." Typical drill string compensators are described in U.S. Pat. Nos. 3,877,680 and 3,804,183. Another type of motion compensator is commonly referred to as a "riser tensioner," and examples of such devices are described in U.S. Pat. Nos. 3,908,963 and 3,314,657.
Drill string compensators, riser tensioners, and other types of offshore motion compensators have certain features in common. Perhaps the most basic of these is the fact that, in any such compensator, there are two bodies interconnected for relative movement, usually telescopic movement, in generally vertical directional modes. These bodies may, for example, be a piston and cylinder. One of the bodies, usually the piston or its attached piston rod, is connected to an offshore well structure such as a drill string or a string of riser pipes, while the other is connected to an offshore support structure, such as drilling platform or vessel. The well structure and support structure are, of course, movable with respect to each other. Such movements will result in corresponding movement between the two compensator bodies. Some means is associated with the two compensator bodies for resiliently resisting relative movement therebetween in a first directional mode. This means may typically be a compressible fluid, usually a gas, interposed between the piston and one end of the cylinder.
It is conventional practice to associate with each such motion compensator a position indicator which provides the operator with an indication of the relative positions of the compensator bodies whereby the movements therebetween can be observed and/or recorded. In this manner, it can be determined, for example, whether or not any adjustment in the apparatus is required from time to time. In the past, the position indicators have suffered from a number of disadvantages. Not only were they relatively complicated, which in turn made them expensive and difficult to service or repair, but they were also inadequate in terms of the accuracy with which they would determine the relative positions of the compensator bodies, and thus, the connected well and support structures.